To prepare pads and holes on a circuit board to subsequently be solder bonded to components placed on the circuit board a number of processes must be performed on the circuit board. Each circuit board needs to be etched, brushed, rinsed and dried. A coating of flux is applied and then heated to enable the flux to flow and chemically prepare the pads and holes to receive molten solder. Once the circuit board is prepared with flux, solder is applied to the pads and holes on the circuit board. Next, the excess molten solder is removed by blowing it off the circuit board and out of holes on the board and the remaining solder on the board is levelled. Levelling solder has traditionally been carried out with a single hot air knife, as shown in U.S. Pat. No. 4,600,137 to Comerford and U.S. Pat. No. 4,410,126 to O'Rourke which are incorporated herein by reference. The circuit board is then cooled down, the solder flux is removed, and the circuit board is physically scrubbed, rinsed and dried to remove any contaminants.
Although the solder removal and levelling process described above does remove some excess solder and helps distribute the solder more evenly, serious solder defects still occur. In particular, the failure of the above-described process to remove excess solder and level the of solder has resulted in undesired solder bridging between pads and in inconsistent distribution of solder thickness producing "starved" solder regions and "frozen solder waves."
One such frozen solder wave 10 which is known in the prior art is shown in FIGS. 1(a)-(b). The solder wave 10 extends up from the solder 12 deposited on the copper pads 14 on the circuit board 16. The frozen solder wave 10 is also referred to as "the double hung window effect" because of its appearance as shown from the top view in FIG. 1(b) and is caused by improper levelling of the pads surrounding holes. Ideally, the solder thickness or height around the holes would be the same as at other locations for the pads on the circuit board 16. The inadequate distribution of the solder which results in the starved regions and frozen solder waves is problematic because it impedes precise placement of surface mount components and may cause solderability problems.